GUADALAJARA, Mexico — Mexico's two most powerful criminal gangs are locked in a titanic battle for control of the country's heartland in a struggle that's redrawn Mexico's map of violence.

Violence has dropped along the U.S. border, with Ciudad Juarez, once considered the most violent city in the world, seeing a 35 percent decline in homicides this year.

That good news is balanced by bad news in Guadalajara, Culiacan and Veracruz, where the Sinaloa cartel, whose bulwark has always been Mexico's Pacific coast, and the Zetas, a violent gang that originally was created to protect the Gulf cartel along the Gulf of Mexico coast, are locked in a spiraling struggle that's seen each gang invade the other's territory.

The conflict has thrust Guadalajara, an important manufacturing center of 4.4 million people, into the battlefield. After overcoming a spate of drug violence in the mid-1980s, Guadalajara quieted down, perhaps because the Sinaloa cartel held a monopoly on operations in the surrounding state of Jalisco.

"Here in Jalisco, we've seen this as a distant thing. 'Oh, this is happening over in Michoacan.' It felt like it was far away," said Dante Haro Reyes, a law professor and public security expert at the University of Guadalajara. "Now it feels like it's around the corner."

The wakeup call came at daybreak last Thursday, when mobsters abandoned three vehicles filled with 26 dead bodies at the iconic bright-yellow Millennium Arches that straddle a Guadalajara thoroughfare. A message on a poster board was signed "Z," a signature of Los Zetas.

"Look how we leave you these dead people," the poster said in part. "We are in your kitchen."

Boasting of their penetration deep into Sinaloa turf, the Zetas claimed to be "the strongest cartel at the national level, the only cartel that doesn't pass information to the gringos," a reference to the son of a Sinaloa boss who claims to have been a Drug Enforcement Administration informant before his 2009 arrest.

Just a day earlier, the Zetas had dealt another blow to Sinaloa, leaving a truck filled with 16 charred bodies in Culiacan, the capital of Sinaloa state, from which the Sinaloa cartel takes its name.

The war between the groups — clearly the alpha dogs of Mexico's underworld — pits not just weapons but also two very different business models and geographic strongholds.

"This is a kind of death struggle, a definitive struggle between the Zetas, who have no remorse and expand constantly, and Sinaloa, which is trying to consolidate itself," said Bruce Bagley, an organized crime and narcotics expert at the University of Miami.

Sinaloa operatives appear to have set off the conflict over the summer, forming a group called "Matazetas," or Zeta Killers, to exterminate Zetas in Veracruz, a Gulf Coast state that's a bottleneck on a key smuggling route. The group went public in a big way at afternoon rush hour on Sept. 20, parking three vehicles packed with dead bodies near an urban underpass. Security agents found 35 victims at the grisly scene, nearly all asphyxiated and partially naked.

The "Zeta Killers" released videos of masked gunmen promising to hunt down Zetas and end their rampant extortion in Veracruz against common people.

Even as they execute plenty of their own rivals, Sinaloa bosses are thought to detest the brutality of the Zetas, which they think brings increased law enforcement pressure on crime groups.

"The Matazetas quite clearly tried to win a kind of public approval and government tolerance. They said, 'Get out of our way and we'll take care of this problem,' " Bagley said.

With the latest Zetas blows against Sinaloa, experts say tit-for-tat violence is taking on its own momentum.

"The theory going around is that this is a battle for total control," Haro Reyes said, adding that reprisals wouldn't take long to occur. "When you get attacked on your own territory, you've got to attack in your rival's territory or you look weak."

Sinaloa and the Zetas have vastly different histories.

Smugglers from Sinaloa began packing marijuana northward half a century ago. Today, the Sinaloa cartel's tentacles loop as far as Australia and West Africa, making it the most powerful drug syndicate in Mexico, and perhaps the world. The group, which is also known as The Federation, is loosely organized and more inclined to negotiate with rivals and bribe authorities.

In comparison, the Zetas are upstarts. A militia formed by former Mexican special forces commandos recruited to protect the Gulf Cartel, the Zetas broke away early last year. Unlike the Sinaloa crime group, which sticks largely to drug trafficking, the Zetas branched into extortion, kidnapping, human smuggling and the sale of pirated goods. Brutality and beheadings have become their hallmark.

Only a year or two ago, Mexico had half a dozen significant crime groups, including the Tijuana, Juarez, Beltran Leyva and La Familia Michoacana cartels. Security forces have crippled some of those groups through arrests and killings, while others have splintered, leaving remnants to struggle for allies.

One of those fragmentations occurred in Guadalajara after the slaying of Sinaloa boss Ignacio Coronel on July 29, 2010. Some of his enforcers have allied with another group, Milenio, and moved under the umbrella of Los Zetas.

If the Zetas win control of Jalisco state, their territory would bisect Mexico, stretching from Tamaulipas along the Gulf Coast through San Luis Potosi and into Jalisco, giving them access to Manzanillo, the nation's busiest port.

While body dumps are becoming common in central Mexico, residents of Ciudad Juarez, where homicides have dropped this year, are finding unusual periods of calm. For 65 hours over Nov. 19 to 21, Juarez tallied no homicides at all, the longest such period in three years.

"There are clear signs of Ciudad Juarez's recovery," Gov. Cesar Duarte of Chihuahua state said last week. "Instead of streets congested with security forces, we have restaurants congested with clients."

To be sure, Ciudad Juarez has tallied 1,832 killings so far this year, an unacceptable rate of about 5.5 homicides day. But the trend line heartens residents.

Ciudad Juarez's police chief, Julian Leyzaola, a former army lieutenant colonel who gained notoriety for tough tactics in quelling crime in Tijuana in an earlier posting there, notes that the drop in murders coincides with his arrival in March.

There may be other reasons, however. The Sinaloa Cartel appears to have reached a settlement with onetime rival cartels in Tijuana and Juarez, negotiating a 60-40 split in drug trafficking profits, "with Sinaloa taking the lion's share," Bagley said.

The agreements may explain why Ciudad Juarez and border areas to the west all the way to Tijuana on the Pacific coast have seen violence drop, he said.